The naphthalenedicarboxylic acid and its ester, particularly 2,6-naphthalenedicarboxylic acid (hereinafter referred to as "2,6-NDCA") and its ester are useful compounds for preparing high performance polyesters. Conventionally, 2,6-NDCA has been prepared by oxidizing 2,6-dialkylnaphthalene, 2,6-diacylnaphthalene or their derivatives in a solvent containing a lower aliphatic carboxylic acid in the presence of a catalyst containing cobalt, manganese and bromine. Such a process is disclosed in Japanese Patent Publication No. 56-3337, Japanese Patent Application Laid-Open No. 60-89445, U.S. Pat. No. 5,183,933, etc.
Unlike the preparation of terephthalic acid by the oxidation of p-xylene, the oxidation of the dialkylnaphthalene is generally accompanied by a remarkable by-production of benzotricarboxylic acid, particularly trimellitic acid (hereinafter referred to as "TMA") is by-produced from 2,6-naphthalenedicarboxylic acid due to the cleavage of the naphthalene ring. In addition, the by-produced benzotricarboxylic acid such as TMA, etc. forms, together with cobalt, manganese, etc. in the heavy metal catalyst, a complex sparingly soluble in the solvent comprising a lower aliphatic carboxylic acid, thereby deactivating the heavy metal catalyst. The decreased amount of the effective heavy metal catalyst due to the deactivation further increases the by-production of TMA, this in turn promoting the deactivation of the heavy metal catalyst. With such a vicious circle, the oxidation reaction is ceased in the worst case.
To avoid the deactivation of the heavy metal catalyst due to the accumulation of the by-produced TMA by recycling the mother liquor, proposed is a method of increasing the concentration of the heavy metal catalyst such as cobalt, manganese, etc. so as to compensate for the decrease of the effective amount due to the formation of the TMA-heavy metal complexes (U.S. Pat. No. 5,183,933, Japanese Patent Application Laid-Open No. 7-48314, etc.). In particular, U.S. Pat. No. 5,183,933 teaches that it is advantageous to use as much manganese as possible because manganese is less expensive than cobalt. However, the use of a greater amount of the heavy metals contaminates 2,6-NDCA crystals with a large amount of the TMA-heavy metal complexes to increase the content of the heavy metals in the crystals. The incorporation of the heavy metals into the crystals is a loss of the catalyst, and causes clogging of conduits in the purification process of 2,6-NDCA.
Several methods have been proposed to remove the incorporated TMA-heavy metal complexes in 2,6-NDCA crystals and recover the heavy metal catalyst. For example, Japanese Patent application Laid-Open No. 1-121237 discloses a method utilizing the relatively high water solubility of the TMA-heavy metal complexes, where 2,6-NDCA crystals are washed with water and the heavy metals in the washings are recovered in the insoluble carbonate forms by adding to the washings a compound forming carbonate ions. U.S. Pat. No. 5,183,933 teaches to add water to the oxidation reaction mixture to increase the water content in the low molecular weight carboxylic acid solvent, thereby dissolving the TMA-heavy metal complexes. Then, 2,6-NDCA crystals are separated from the solvent by a solid-liquid separation.
However, the method of Japanese Patent Application Laid-Open No. 1-121237, where the heavy metals incorporated in 2,6-NDCA are recovered by washing with water, is not practical for industrial scale process because the waste water after recovering the heavy metals requires expensive treatment for removing organic compounds such as TMA dissolved therein. The method of U.S. Pat. No. 5,183,933 is energy-consuming because water must be removed from the mother liquor before the catalyst and the lower aliphatic carboxylic acid are recovered and reused.
In the known method of producing 2,6-NDCA by the oxidation of 2,6-dialkylnaphthalene, as mentioned above, an excessive amount of the heavy metal catalyst must be used because the heavy metal catalyst is deactivated by the by-produced TMA, this causing in turn a large amount incorporation of TMA-heavy metal complexes into 2,6-NDCA crystals. Further, since the recycling of the mother liquor accumulate TMA in the reaction zone, much more excessive amount of heavy metal catalyst is required to result in the incorporation of a significantly increased amount of TMA-heavy metal complexes into 2,6-NDCA crystals. Since the recovery of the heavy metal catalyst incorporated in 2,6-NDCA requires a great amount of water, the heavy metal catalyst components and the solvent (lower aliphatic carboxylic acid) cannot be efficiently recovered.
The known method of producing the naphthalenedicarboxylic acid has encountered another problem. Since the crystals of naphthalenedicarboxylic acid formed by the oxidation reaction has an extremely small particle size, a solid-liquid separation for separating the crystals and solvent such as a centrifugal sedimentation, a centrifugal filtration and a suction filtration usually employed in the industrial process involves problems of a slow sedimentation or a low filtration speed, causing an increased amount of crystals remaining in the mother liquor, a clogging of filter, etc.
To facilitate the separation, several methods where the oxidation reaction was conducted under specific conditions for forming naphthalenedicarboxylic acid crystals having a larger particle size have been proposed. For example, a method of conducting the oxidation reaction at a specific temperature range of 180 to 220.degree. C. (Japanese Patent Application Laid-Open No. 6-65143), a method of conducting the oxidation reaction under a low oxygen concentration (Japanese Patent Application Laid-Open No. 8-143511), and a method of adding an ester mixture containing dimethyl naphthalenedicarboxylate (Japanese Patent Application Laid-Open No. 8-193049) have been known.
In the above methods of conducting the oxidation reaction under specific conditions to form naphthalenedicarboxylic acid crystals having a larger particle size, the particle size achieved is, at most, about 40 .mu.m and the improvement in facilitating the solid-liquid separation by increasing sedimentation and filtration speed and avoiding the crystals from remaining in the mother liquor is insufficient.
Thus, an object of the present invention is to provide a method of producing naphthalenedicarboxylic acid by oxidation reaction of dialkylnaphthalene suitable for industrial-scale process and that can efficiently recover the heavy metal catalyst and solvent.
Another object of the present invention is to provide an industrially advantageous method of producing naphthalenedicarboxylic acid by oxidation reaction of dialkylnaphthalene to form naphthalenedicarboxylic acid crystals having a larger particle size capable of facilitating the separation and recovery of the crystals.